1. Field of the invention.
The present invention relates to an apparatus and a method for increasing the throughput of a rapid thermal processing system. More specifically, the present invention discloses a chamber with an entry aperture of reduced area for an object to be processed, and a method of gas flushing a rapid thermal processing heating chamber which allows a shorter time for flushing between the time the object has been introduced into the chamber and the door closed and the time the heating of the object starts. The reduced aperture and a specific set of steps for flushing the chamber with a gas, most importantly a reduced time for a preliminary flushing step using a gas flow with a flow rate which is low enough that the flow is laminar, increase the throughput of the system and decrease the process gas consumption.
2. Description of the prior art.
Rapid Thermal Processing (RTP) is a versatile optical heating method which can be used for semiconductor processing as well as a general, well controlled, method for heating objects or wafers which are in the form of thin sheets, slabs, or disks. The objects are inserted into a chamber which has at least some portions of the chamber walls transparent to transmit light from powerful heating lamps. The light from the lamps is directed through the transparent portions of the walls on to the surface of the object to be heated. As long as the objects absorb light in the infrared or visible spectral region, RTP techniques allow fast changes in the temperature and process gas for the different material processes and conditions. RTP allows the "thermal budgets" of the various semiconductor processing to be reduced, as well as allows the production of various metastable states which can be "frozen in" when the material is cooled rapidly.
RTP systems are relatively new. In the last 10 or 15 years, such systems were used only in research and development. The thrust of the work was increasing the temperature uniformity, and developing heating cycles and processes which decreased the thermal budget. However, now that the systems are to be used in manufacturing, the issue of throughput becomes much more important. Higher heating powers, faster material handling, and more effective cooling of the material are now the major problems of interest.
A factory requires that only about 90-180 seconds are spent on each wafer in an RTP system. Of this time, the times for heating and cooling and dwell times are specified by the process technology and can not be changed. One of the places to save time is the step necessary in most processes where the RTP chamber is pre-flushed after the wafer is introduced and the door closed. This pre-flushing step is necessary to remove atmospheric impurities which enter the chamber each time the door is opened. When titanium or cobalt is deposited on the wafer and rapidly heated to form titanium or cobalt silicide, for example, traces of oxygen will react with the metals and ruin the process. Nitrogen or another inert process gas is generally introduced into the chamber to prevent this. The shortest time that has been heretofore used in a commercial RTP reactor for pre-flushing with the process gas was 30 seconds. This time could only be reached with very good gas distribution in the chamber and a relatively high flushing rate.
Most RTP machines have a thin rectangular quartz reaction chamber having one end open as sketched in FIG. 1. Chambers meant for vacuum use often have a flattened oval cross section. Chambers could even be made in the form of a flat cylindrical pancake. In general, the chambers are used so that the thin objects to be heated are held horizontally, but they could also be held vertically. The reactor chamber is usually as thin as possible to bring the lamps as close as possible to the object to be heated. The reactor chamber is opened and closed at one end with a pneumatically operated door when the wafer handling system is in operation. The door is usually made of stainless steel, and may have a quartz plate attached to the inside. The process gas is introduced into the chamber on the side opposite the door and exhausted on the door side. The process gas flow is controlled by computer controlled valves connected to various manifolds in a manner well known in the art.
All reactors based on this principle have the entire cross section of one end of the reactor chamber open during the wafer handling process. This construction has been established because the various wafer holders, silicon rings, and gas distribution plates, which are significantly bigger and may be thicker than the wafers, must also be introduced into the chamber and must be easily and quickly changed when the process is changed or when different wafer sizes, for example, are used. The reaction chamber dimensions are designed with these ancillary pieces in mind.
We have recognized that each time the warm chamber is opened a horizontal chimney effect is established. The warm gas exits from the chamber in a flow that is concentrated at the top of the chamber. The exit velocity of the gas is higher, and by the Bernoulli principle the lowered pressure pulls in cold gas from the outside in a flow along the lower part of the chamber. This impurity source in the chamber is the origin of the heretofore necessary long preliminary flushing time.
After the object to be processed has been introduced into the chamber and the door closed, the impurities are concentrated near the door. Prior art methods of flushing used high flow rates, so that the flow "bounced off" the door and back into the chamber so that the impurities were uniformly distributed throughout the chamber. The time taken to dilute the impurities to an acceptable level is then quite long, and the flow rates of process gas required were quite high.